The two planes were so close to that their traffic collision avoidance alarm went off.

NEW DELHI -- In a near miss incident, two aircraft came close to each other in Goa airspace, triggering the planes' traffic collision avoidance alarm system to go off and forcing their pilots to take necessary action to maintain a safe distance between them.

The incident, involving a SpiceJet's Hyderabad-bound plane from Goa and a GoAir aircraft operating on Nagpur-Mumbai route, took place on Sunday and a probe has been ordered by the civil aviation regulator -- Directorate General of Civil Aviation (DGCA), sources said.

According to sources, SpiceJet's Bombardier Q400 plane, operating flight SG-3604 from Goa to Hyderabad while in the climb in Goa airspace, experienced a Traffic Collision Avoidance System (TCAS) warning when it breached the minimum required vertical separation distance with GoAir flight (G8-141).

The crew took evasive action as directed and reported to Goa air traffic control, they said.

When contacted, both SpiceJet and GoAir officials confirmed the incident but said it was being investigated by the DGCA.

TCAS is an onboard system that alerts pilots about the proximity of an aircraft and also instructs them about the avoidance action (Resolution Advisory) to take necessary action to maintain desired separation between two aircraft.

In the Indian airspace, two planes can fly at minimum permitted vertical separation of 1,000 feet, and a breach in this separation prompts the TCAS to go off.